Fusobacterium nucleatum is a Gram-negative anaerobe implicated in various forms of periodontal diseases. It is also associated with infections in other parts of the body and is one of the most prevalent species in intra- amniotic infection, causing preterm birth. F. nucleatum binds to and invades host epithelial and endothelial cells, a mechanism allowing colonization at different host sites. Previous studies have shown that F. nucleatum can translocate to the pregnant mouse placenta via haematogenous transmission, followed by activation of localized placental inflammatory responses, leading to adverse pregnancy outcomes. Invasion of mouse placental endothelial cells by F. nucleatum has also been observed in vivo. So far, only one adhesin, FadA (for Fusobacterium adhesin A), has been identified to be required for bacterial binding and invasion of host cells in both tissue-culture and animal models. FadA is a unique adhesin consisting of two forms: the intact non-secreted form (pre-FadA) composed of 129 amino-acid (aa) residues, and the mature secreted form (mFadA) of 111 aa. The crystal structure of mFadA reveals two anti-parallel alpha-helices connected by an 8- aa loop. The crystal structure of mFadA suggests oligomerization in a head-to-tail pattern via a novel leucine chain motif. Filament formation and binding to host cells require both pre-FadA and mFadA. A FadA adhesin model has been proposed, with pre-FadA anchored in the inner membrane and a chain of mFadA on top of pre-FadA protruding through the outer membrane. We hypothesize that the receptor-binding site may be located in the loop region, only fully exposed at the tip of the filament. Furthermore, several of the FadA filaments may bundle together to form a cluster of loops, which may be required for binding. One focus of this proposed study is to test the FadA adhesin model. Using a yeast-two-hybrid system, several putative receptors have been identified to interact with FadA. Thus, a second focus of this study is to validate the interactions between the putative receptors and FadA and to investigate the host responses to FadA. Our specific aims are: Aim I. Further characterization of the FadA adhesin in F. nucleatum. Five sub-aims are proposed: (i) investigating fadA expression under different conditions, (ii) investigating the spatial arrangement of FadA in F. nucleatum, (iii) investigating the involvement of the loop region in host-cell binding, (iv) investigating the role of the signal peptide in FadA complex formation, and (v) investigating possible accessory molecules in F. nucleatum associated with FadA. Aim II. Investigation of FadA and host cell interactions. Two sub-aims are proposed: (i) continued identification and characterization of the FadA receptor, and (ii) investigating the effect of FadA on cellular processes. From this study, we hope to identify (i) potential therapeutic targets for inhibiting F. nucleatum colonization in the host, and (ii) host components and pathways affected by FadA, which will facilitate modulation of respective cellular processes and targeted drug delivery.